A shock absorber is installed to a vehicle body to support the weight of the vehicle body and reduce vibration transmitted from a road to the vehicle body. In addition, the shock absorber absorbs vertical vibration energy of wheels caused by irregularity of a road and the like, thereby contributing to enhancing comfort of passengers and protection of cargos and parts of a vehicle.
In general, a shock absorber comprises a cylinder connected to a wheel and a piston rod connected to a vehicle body. An interior of the cylinder is filled with oil, and the piston rod is connected to a piston valve, which is provided in the cylinder and divides the interior of the cylinder into a rebound chamber and a compression chamber. The piston valve comprises a rebound flow passage and a compression flow passage, which connect the rebound chamber and the compression chamber. When the piston valve is moved, oil passes through the aforementioned flow passages and a certain damping force is generated.
In the meantime, pressure in the compression chamber is changed when the piston valve is moved, the shock absorber has a structure for compensating the pressure in the compression chamber to cope with such pressure change. In addition, according to the structure for compensating the pressure, shock absorbers are classified into a telescope-type shock absorber having a cylinder consisting of one shell and a twin-shell type shock absorber having a cylinder with a dual structure having inner and outer shells.
In the twin-shell type shock absorber, an inner space of the inner shell is filled with oil and divided into a rebound chamber and a compression chamber by a piston valve. A reservoir chamber filled with oil is provided inside of the outer shell surrounding the inner shell. In addition, a body valve is provided at a portion at which the inner shell and the outer shell are connected to each other. The body valve is formed with a flow passage for connecting the compression chamber and the reservoir chamber. The pressure in the compression chamber of the shock absorber can be compensated by oil flowing from the reservoir chamber to the compression chamber through the flow passage when the piston valve is moved upward, i.e., in the rebound stroke of the shock absorber. When the piston valve is moved downward, i.e., in the compression stroke of the shock absorber, the body valve generates generally a damping force.
However, the conventional shock absorber has a problem in that the “lag” phenomenon is generated below the piston valve, i.e., in the compression chamber when the piston valve is moved upward due to the rebound stroke. Such a “lag” phenomenon is caused by delay of a time for which oil having flowed into the reservoir chamber by the compression stroke of the shock absorber flows from the reservoir chamber to the compression chamber when the compression stroke is shifted to the rebound stroke. This “lag” phenomenon causes a serious distortion of the damping force in the compression stroke of the shock absorber. As one scheme for preventing the “lag” phenomenon, a structure in which a cross-section of the flow passage of the body valve is enlarged has been examined. However, there is a limit to an enlargement of cross-section area of the flow passage from the reservoir chamber to the compression chamber in the body valve that requires the bidirectional flow passage and has a defined cross section area.
In addition, in the conventional shock absorber, as an operating time is lengthened, the gas pressure in the reservoir chamber drops below a certain value, so that the oil cannot be sufficiently supplied to the compression chamber in the rebound stroke, which may also cause the “lag” by which distortion of the damping force is caused.